1. Field of the Invention
Referring to FIG. 1, the present invention relates to cleaning layers of strained silicon (sSi) 10 denuded by selective etching of a layer of silicon-germanium (SiGe) 12 in contact with the sSi layer 10, as used in the fabrication of sSOI (strained silicon-on-insulator) or SGOI (silicon-germanium (SiGe)-on-insulator) type wafers.
2. State of the Art
A number of techniques exist for preparing such wafers. One example of one of the most effective current techniques for fabricating sSOI- or SGOI-type wafers is the production of an active layer of strained silicon (sSi) or relaxed SiGe by transfer thereof onto an insulating support (for example, a layer of SiO2 14 on a silicon substrate 16) using the SMARTCUT® technique to produce the desired heterostructure. Particular examples of implementations of such fabrication techniques are described in U.S. Patent Application Publication No. 2004/0053477 and International Patent Publication WO 2004/006311.
The finishing step of such wafers involves a selective etching procedure to eliminate the SiGe layer 12 subsisting above the silicon layer 10 after transfer thereof to the “receiver” substrate 16 and detachment from the “donor” substrate (not shown). Selective etching is a chemical attack method that can selectively eliminate the upper layer of SiGe 12 without attacking the next layer of strained silicon (sSi) 10. Compositions that have been found to be capable of achieving such a “selective etch” include CH3COOH/H2O2/HF mixtures.
After the selective etch step, which removes the upper layer of SiGe 12, a cleaning step may appropriately be used in order to reduce particulate contamination and surface defects on the surface 11 of the strained silicon layer following selective etching of the SiGe layer 12. The cleaning composition must thus be chosen so as to enable cleaning of the sSi surface 11 while avoiding etching of this strained silicon layer 10.
This post-selective etch cleaning step generally uses compositions containing oxidants, but not containing hydrofluoric acid (HF). A known method, termed “RCA,” comprises: a first step of cleaning using an “SC1” (Standard Clean 1) solution (or APM, ammonium hydroxide peroxide mixture) containing NH4OH, H2O2, and deionized water, and a second cleaning step using an “SC2” (Standard Clean 2) solution (or HPM, hydrochloric peroxide mixture) containing HCl, H2O2, and deionized water.
An alternative treatment method, proposed in U.S. Patent Application Publication No. 2006/0264008, comprises a step of cleaning the surface 11 of the strained silicon layer 10 with an aqueous ozone solution, rather than using the RCA cleaning procedure, following the step of selective etching of the SiGe layer 12.
The wafer obtained by the process steps of 1) selective etching of the SiGe layer 12, and 2) oxidative cleaning, may then be used in subsequent semiconductor device manufacturing processes. Generally, the oxidative cleaning step is followed by a step of rinsing with deionized water.
The wafer with an exposed sSi 10 thus obtained may be used in MOS transistors. Workers in the semiconductor industry are in effect constantly engaged in an effort to reduce the dimensions and increase the speed of operation of integrated circuits. Other parameters being equal, the operation speed of a transistor is increased by increasing the mobility of charge carriers, both electrons and (positive) holes, and one way of achieving this is by the use of strained silicon 10, wherein silicon, grown on a silicon-germanium surface, is obliged to have more broadly spaced atoms than in pure silicon. A doubling of charge carrier mobility with respect to normal, relaxed silicon can be observed.
Concerning the existing technology in this area, the present inventors have found that the degree of defects as measured by SOD (sum of defects) remains higher than is desirable for the applications envisaged for the wafers obtained.
In addition, the present inventors have observed that the SiGe 12 is not completely removed by prior art processes. Instead, although SiGe 12 is completely removed in certain areas, other zones, or “islands,” of SiGe 12 persist. It has been observed that these islands may correspond to about 35% of the surface defects measured. Germanium atoms from such islands may diffuse into and contaminate the strained silicon layer 10. Further, these islands may disturb subsequent fabrication steps, e.g. doping steps.
There therefore remains a need in the art to produce sSi layers 10 showing a minimum amount of surface defects and a minimum amount of residual SiGe 12. Any further cleaning processes other than prior art processes of 1) selective etching of the SiGe layer 12, and 2) oxidative cleaning, need, however, to be chosen so as to minimize consumption of now exposed parts of the sSi layer 10.